The present invention relates to apparatus for scrambling digital video data and, more particularly, apparatus for intermittently scrambling or partially scrambling digital video data where in the intermittent mode each frame is or is not scrambled depending upon its frame number, and in the partial scramble mode each block of data in each frame is or is not scrambled or not scrambled depending upon its block number.
It is known that when an analog video signal representing a video image is transmitted, the video image formed by the information contained in the received video signal is of a lesser quality than the original video image. This usually is due to noise, reduction in the signal to noise ratio, etc. In order to solve this problem to improve the quality of the video signal that is received, it is known to convert the analog video signal to a digital video signal prior to transmission. The digital video signal usually is encoded in a predetermined format prior to transmission and then decoded back to its original non-encoded form.
One example of such a format is shown in FIG. 1. As shown, each video frame of video data of the encoded signal includes a frame header followed by blocks of video data, wherein each video data block includes a block header and block data. Image information provided in the original video signal is stored in the block data portion of each data block. Each frame header and block header includes information such as synchronizing data, frame number, line number of a frame, etc.
In addition to digitizing and encoding a video signal prior to transmission, the video signal may also be compressed in order to reduce the amount of data to be transmitted, which results in more efficient transmission of video signals. One method of compressing data is interframe coding wherein the difference between pixel values or block values of the current video frame and the preceding video frame is calculated to produce a resultant interframe-coded frame of video data. Data in other video frames (e.g., succeeding frames) may also be utilized to generate each interframe-coded frame.
When a video signal is interframe coded and then transmitted, the received signal must be decoded in conformity with the interframe coded format. That is, when decoding each interframe-coded frame, information from other frames must also be utilized and decoded in order to decode the successive, received frame. One problem that exists when transmitting interframe-coded signals is the possibility of the occurrence of an error during transmission of the encoded video signal. Since each interframe-coded frame is directly related to other frames, errors that occur in one frame propagate to all frames that are influenced by (i.e., dependent upon directly or indirectly) the frame in which the error occurred. This results in the loss of multiple frames of data even though the original error occurred only in a single interframe-coded frame. This "chain-like" influence results in a substantially damaged video signal.
To reduce the effect of the above described error propagation, it is known to include so-called frame-limit coded frames of video data with the transmitted interframe coded frames. Each frame-limit coded frame is formed by frame-limit coding only one frame of video data such that upon receipt of the frame-limit coded frame, the video data corresponding to one frame may be reconstructed by decoding only the frame-limit coded frame without the need for obtaining information from any other frame.
When transmitting frame-limit coded frames with interframe coded frames, a single frame-limit coded frame is first transmitted followed by a predetermined number of interframe-coded frames, wherein each of the interframe-coded frames are related to (dependent upon) that frame-limit coded frame. For example, upon receiving the coded signal, the second video frame is reconstructed by utilizing the data in the frame-limit coded frame (the first transmitted frame) and the first interframe coded frame (the second transmitted frame); the third video frame is reconstructed by utilizing the data in the first interframe coded frame (the second transmitted frame) and the second interframe coded frame (the third transmitted frame), and so on. After a predetermined number of interframe-coded frames are transmitted, the cycle repeats, wherein a frame-limit coded frame is transmitted, followed by the predetermined number of interframe coded frames. Thus, errors which occur in a frame during transmission will affect only the successively transmitted frames up to, but not including, the next frame-limit coded frame.
It is sometimes desirable to transmit scrambled video data, one example being the commercial benefit of scrambling video signals by a cable broadcasting station, e.g., the broadcast of a program for the commonly known "Pay Per View" channels. When scrambled video data is transmitted, a subscriber (i.e., viewer) who receives a "scrambled" channel must have a descrambling device which descrambles the received video signal in order to be able to view the broadcast program. If the scrambled video signal were viewed (e.g., on a video monitor) without first being descrambled, the image would be unrecognizable.
Three types of scrambled video signals are known as fully scrambled video signals, intermittently scrambled video signals and partially scrambled video signals. The fully scrambled video signal is completely scrambled, that is, each and every frame of video data is scrambled and each portion (e.g., block) of each frame is scrambled. When descrambling a fully scrambled video signal, each and every frame must be descrambled for the image to be totally recognizable and if any frames are not descrambled, the corresponding images would be unrecognizable. The intermittently scrambled video signal, which is scrambled in an "intermittent scrambling mode", includes scrambled frames of video data and non-scrambled frames of video data such that a descrambling device should only descramble those frames that have been previously scrambled. If none of the frames of an intermittently scrambled video signal are descrambled, the video image would be intermittently recognizable when viewed on a monitor. The partially scrambled video signal, which is scrambled in a "partial scrambling mode", contains frames of video data partially scrambled, that is, a portion of each frame is not scrambled and, thus, is recognizable when viewed even in the absence of descrambling. Of course, that portion of the frame which is scrambled is unrecognizable unless it is descrambled.
When video data is frame-limit coded and interframe coded as previously described, and also intermittently scrambled or partially scrambled, it is necessary (upon receipt of the scrambled signal) to descramble only selected portions of the video signal (i.e. those portions which have been scrambled) in order for the video signal to be properly descrambled and viewed. When frame-limit coded frames and interframe coded frames are intermittently scrambled prior to transmission, a receiving device must descramble only those frames that were scrambled prior to transmission. However, such a descrambling device is extremely complex since it is difficult to determine whether a frame has been scrambled or not, and also, since interframe coded frames are dependent upon other frames, there may be frames that are dependent upon both scrambled and non-scrambled frames, which adds to the complexity of such a system.
Similarly, when frame-limit coded frames and interframe coded frames are partially scrambled prior to transmission, blocks of video data of each interframe coded frame, which similarly may be scrambled or not scrambled, are dependent upon blocks of data within other frames, which likewise may or may not be scrambled, thus requiring complex calculations to properly descramble the signal. Further blocks of data within a specific frame may also be dependent upon other blocks of data within the same specific frame, which again, may or may not be scrambled.
In addition to the above described problems, intermittently scrambling and partially scrambling frame-limit coded and interframe coded video signals removes some of the advantages associated with transmitting intermittently scrambled and partially scrambled video signals. For example, during transmission of a video signal, it may be desirable to have a portion of that video signal recognizable (e.g., by a viewer). One example of such a case is the desire to entice a viewer to pay a fee in order to view the transmitted signal fully descrambled (e.g., as in the above mentioned pay-per-view channel example). Ordinarily, when intermittently scrambled video signals are viewed without being descrambled, there may be, for example, several, seconds of non-scrambled images at various times of transmission. Similarly, partially scrambled signals allow a viewer to recognize a small (or large) portion of each image. However, these advantages and other advantages of transmitting intermittently scrambled or partially scrambled video signals disappear when the video signals are frame-limit coded and interframe coded prior to transmission.